Rationally designed a g-C₃N₄/BiOI/Bi₂O₂CO₃ composite with promoted photocatalytic activity

Highlights

• A novel ternary g-C₃N₄/BiOI/Bi₂O₂CO₃ photocatalyst was successfully prepared.

• The g-C₃N₄/BiOI/Bi₂O₂CO₃ showed the highest adsorption capability and photocatalytic activity.

• The h⁺ and .^•O₂⁻ were the main active radicals in the photocatalystic process over g-C₃N₄/BiOI/Bi₂O₂CO₃.

• Relying on the matched energy levels and band gaps, BiOI acted as the charge transmission-bridge.

• The enhanced photocatalytic activity stems from the effective separation of photoinduced e^––h⁺ pairs.

Abstract

It is vital for environment purification to design and prepare an artificial photocatalyst that can effectively separate photogenerated carriers and maintain the redox ability of system. In this paper, a g-C₃N₄/BiOI/Bi₂O₂CO₃ photocatalyst was rationally designed and successfully prepared via two-step in-situ transforming process. The results of X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy confirm the successful synthesis of ternary g-C₃N₄/BiOI/Bi₂O₂CO₃ catalysts. The photocatalytic activities of all catalysts were assessed by the degradation of Rhodamine B (RhB) under simulated sunlight illumination. The g-C₃N₄/BiOI/Bi₂O₂CO₃ catalysts showed much higher photocatalytic activities than single g-C₃N₄, or BiOI and dual g-C₃N₄/BiOI. The results of electron spin resonance and active species trapping experiments revealed that h⁺ and ^•O₂⁻ were responsible for the photodegradation of RhB, and a possible photogenerated carrier transport mechanism was proposed.

Introduction

The semiconductor photocatalytic technology has a great prospect in the photocatalytic degradation of organic pollutants and the generation of green energy [1]. The semiconductor photocatalysts are the key factor in the development of efficient photocatalytic technology, which includes three central scientific problems: sunlight absorption, charges separation and transfer, and surface catalytic reaction [2]. At present, around the three core issues, a great deal of research has been devoted to exploiting new-style efficient photocatalysts that can make full use of sunlight [[3], [4], [5]]. Among them, graphitic carbon nitride (g-C₃N₄) is a widely studied nonmetal semiconductor photocatalyst because it is stable, non-toxic, economical and easy to prepare [6]. The band gap of g-C₃N₄ is about 2.70 eV, and the conduction band (CB) potential of g-C₃N₄ is comparatively minus (−1.3 eV) [7] that can make photogenerated electrons have stronger reduction ability. Nevertheless, the photocatalytic property of g-C₃N₄ is constrained by the slower transfer and higher recombination ratio of photon-generated carrier and destitute absorption of visible light [8]. Meanwhile, the positive valence band (VB) potential (1.4 eV) [7] of g-C₃N₄ causes the weaker oxidation of photogenerated holes. To overcome these shortcomings, considerable approaches have been utilized to upgrade the photocatalytic activity of g-C₃N₄, such as nanostructures control [9,10], heteroatom doping [11,12] and semiconductor coupling [13,14]. Among these methods, the semiconductor composites overcome the disadvantages of single photocatalyst, and enhance the absorption range of light, the stability and the redox ability [15].

Hitherto, there is a wide concern about bismuth-based photocatalysts because of its efficient use of sunlight, unique electronic properties and superior photocatalytic activities. Multitudinous bismuth-based photocatalysts have been reported, like Bi₂O₃ [16], BiVO₄ [17], Bi₂O₂CO₃ [18], Bi₂WO₆ [19], and BiOX (where X = Cl, Br or I) [[20], [21], [22]]. Among them, BiOX are the significant V-VI-VII semiconductors [23] and own typical Sillén structure staggered by [Bi₂O₂]²⁺ layers and [X^‒] layers [24]. Thereinto, bismuth oxyiodide (BiOI) has strong absorption in the visible-light region because of its narrow band gap (1.7–1.9 eV) [25]. Hence, BiOI exhibits visible-light photocatalytic activity and has been a promising material for environmental improvement. However, its narrow band gap brings about the rapid electron–hole (e^––h⁺) recombination [26]. The efficient strategy to solve the above issue is to combine with other semiconductor materials, which can restrain the recombination of photogenerated e^––h⁺ and promote the charge transfer efficiency, and then greatly enhance the photocatalytic properties [27].

The Sillén group owns a unique layered structure and its general formula is [M₂O₂][Xm] (M = Bi, Ca, Sr, Ba, Pb, and Cd; X represents halogen ions or other ions). The crystalline structure is staggered by [M₂O₂]⁺ layer and [X]^– layer [28]. As another typical member of the Sillén group, the layered structure of Bi₂O₂CO₃ consisting of [Bi₂O₂]²⁺ layers interleaved with CO₃²⁻. Moreover, Bi₂O₂CO₃ owns a more positive VB potential at 3.56 eV [29] for oxidation reactions. However, the band gap of Bi₂O₂CO₃ is wider (2.8–3.4 eV) [30], impeding the effective visible-light absorption and resulting in poor photocatalytic activity. Up to now a series of strategies have been reported to broaden the light absorption range and improve the photocatalytic efficiency of Bi₂O₂CO₃ [31]. In a recent work, we have prepared a flower-like β-Bi₂O₃/Bi₂O₂CO₃ photocatalyst exhibiting an enhanced photocatalytic performance [32]. Based on this satisfactory study, we would like to explore other bismuth materials. Considering that BiOI has strong absorption in the visible-light region, the combination of Bi₂O₂CO₃ and BiOI can strength the effective visible-light absorption. For another thing, because Bi₂O₂CO₃ and BiOI all belong to the Sillén layered structure and have dissimilar solubility product constant, it is easier for them to combine through simple ion exchange [33], thereby bringing about the promoted photocatalytic activity.

In view of the advantages and disadvantages of g-C₃N₄, BiOI as well as Bi₂O₂CO₃ and the band structures of the three, a g-C₃N₄/BiOI/Bi₂O₂CO₃ composite was rationally designed and synthesized though the simple reflux method and in-situ ion exchange method. The photocatalytic ability to degrade Rhodamine B (RhB) pollutant under simulated sunlight irradiation of as-prepared g-C₃N₄/BiOI/Bi₂O₂CO₃ catalyst was investigated. Furthermore, the probable photocatalytic mechanism of the ternary catalyst was eventually discussed.